the importance of the representation of deep convection for modelled dust-generating winds over west...

The Importance of the Representation of Deep Convection for Modelled Dust-Generating Winds over West Africa during Summer J. H. Marsham 1,2 , P. Knippertz 2 , N. Dixon 2 , D. J. Parker 2 and G. M. S. Lister 1,3 1 National Centre for Atmospheric Science, UK. 2 University of Leeds, UK. 3 University of Reading, UK. Contact: [email protected] 10-day Unified Model (UM) simulations suggest that cold-pool outflows from convective storms (“haboobs”) generate ~50% of the dust-generating winds in summertime West Africa. Haboobs are poorly captured in models with parameterised deep convection. To isolate the role of meteorology we define “uplift potential” as U 3 (1+U t /U)(1-U t 2 /U 2 ), where U is 10-m wind and U t =7 ms -1 a typical threshold velocity 2 . Uplift potential would control parameterised dust uplift 1 if the bare-soil land surface was uniform. Simulations with explicit deep convection show haboobs generating ~50% of the uplift potential (Fig 3), but 10-day averages of uplift potential are similar between all simulations (Fig. 2). Over 10 days, the lack of haboobs in models with parameterised convection is compensated by increased uplift from nocturnal low-level jets (LLJ, Fig. 3b) resulting from of a stronger Saharan Heat Low (SHL) in these models (Fig. 4). Only runs with explicit deep convection show significant uplift potential over the Sahel (Fig. 2), which may help explain an observed northwards bias in dust in some regional models 3 . Tuning cannot resolve the problem of haboobs in models with parameterised deep convection – a haboob parameterisation is required. References: 1 Marticorena and Bergametti, JGR,100, 1995. 2 Chomette et al., JGR, 104, 1999. 3 Johnson et al., early view QJRMS, 2011. 4 Pearson et al., JGR, 115, 2010. 5 Knippertz, Met. Zeitschr., 17, 2008. 6 Weisman et al., MWR, 125, 1997. 7 Bou Karam et al., QJRMS, 134, 2008. 8 Marsham et al., JGR, 113, 2008. Paper on this has now been published in GRL!!! The Cascade project simulations Four UM 4 runs for the 10-days 25 July to 03 August 2006: (i) 40-km and 12-km runs with parameterised deep convection (ii) 12-km and 4-km runs with explicit deep convection. Also, a 1.5-km explicit run for the 25 to 26 July. All initialised and forced at boundaries with ECMWF analyses. Fig. 1. UM domains. Main maximum at southern edge of SHL (16 to 22°N) in all runs. Area totals in [ ] similar in all runs. Arcs from haboobs in runs with explicit deep convection (most apparent in the Sahel). 12 km Explicit [15.0x10 5 ] 4 km Explicit [15.4x10 5 ] 12 km Param. [15.1x10 5 ] 40 km Param. [16.3x10 5 ] E.g. Arcs from haboobs E.g. Arcs from haboobs 09–10 UTC mixing of low-level jet (LLJ) to the surface 5 in all runs. Haboob peak in the afternoon accounts for ~50% in explicit runs, is greatly underestimated with parameterised convection and is delayed for coarser grid spacings as expected 6 . 10-day runs: lack of haboobs with parameterised deep convection compensated by stronger SHL (Fig. 4; more LLJs and increased nocturnal monsoon flow 7 ). Haboob peak LLJ peak Haboob peak Dashed=param. Solid=explicit Dashed=param. Solid=explicit Nocturnal monsoon flow 1.5-km domain Fig. 4. The mean SHL Runs with parameterised deep convection develop a deeper SHL and so a stronger LLJ (Fig. 3b), All simulations are within two analyses. More observations from SHL region needed. Explicit cold pools may lead to more effective ventilation of SHL. 40 km param. 12 km param. 12 km explicit 4 km explicit ECMWF analysis NCEP analysis Fig. 2. Ten-day mean uplift potentials Fig. 3. Mean diurnal cycle Mean: 25-26 July Mean: 25 July to 3 Aug. 4-km domain

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The Importance of the Representation of Deep Convection for Modelled Dust-Generating Winds over West Africa during Summer

J. H. Marsham1,2, P. Knippertz2, N. Dixon2, D. J. Parker2 and G. M. S. Lister1,3

1National Centre for Atmospheric Science, UK. 2University of Leeds, UK. 3University of Reading, UK. Contact: [email protected]

10-day Unified Model (UM) simulations suggest that cold-pool outflows from convective storms (“haboobs”) generate ~50% of thedust-generating winds in summertime West Africa. Haboobs are poorly captured in models with parameterised deep convection. To isolate the role of meteorology we define “uplift potential” as U3(1+Ut/U)(1-Ut

2/U2), where U is 10-m wind and Ut=7 ms-1 a typical threshold velocity2. Uplift potential would control parameterised dust uplift1 if the bare-soil land surface was uniform.

Simulations with explicit deep convection show haboobs generating ~50% of the uplift potential (Fig 3), but 10-day averages of uplift potential are similar between all simulations (Fig. 2).

Over 10 days, the lack of haboobs in models with parameterised convection is compensated by increased uplift from nocturnal low-level jets (LLJ, Fig. 3b) resulting from of a stronger Saharan Heat Low (SHL) in these models (Fig. 4).

Only runs with explicit deep convection show significant uplift potential over the Sahel (Fig. 2), which may help explain an observed northwards bias in dust in some regional models3.

Tuning cannot resolve the problem of haboobs in models with parameterised deep convection – a haboob parameterisation is required.

References: 1Marticorena and Bergametti, JGR,100, 1995. 2Chomette et al., JGR, 104, 1999. 3Johnson et al., early view QJRMS, 2011. 4Pearson et al., JGR, 115, 2010. 5Knippertz, Met. Zeitschr., 17, 2008. 6Weisman et al., MWR, 125, 1997. 7Bou Karam et al., QJRMS, 134, 2008. 8Marsham et al., JGR, 113, 2008. Paper on this has now been published in GRL!!!

The Cascade project simulations

Four UM4 runs for the 10-days 25 July to 03 August 2006:

(i) 40-km and 12-km runs with parameterised deep convection

(ii) 12-km and 4-km runs with explicit deep convection.

Also, a 1.5-km explicit run for the 25 to 26 July.

All initialised and forced at boundaries with ECMWF analyses.

Fig. 1. UM domains.

Main maximum at southern edge of SHL (16 to 22°N) in all runs.

Area totals in [ ] similar in all runs.

Arcs from haboobs in runs with explicit deep convection (most apparent in the Sahel).

12 km Explicit[15.0x105]

4 km Explicit[15.4x105]

12 km Param.[15.1x105]

40 km Param.[16.3x105]

E.g. Arcs from haboobs E.g. Arcs from haboobs

09–10 UTC mixing of low-level jet (LLJ) to the surface5 in all runs.

Haboob peak in the afternoon accounts for ~50% in explicit runs, is greatly underestimated with parameterised convection and is delayed for coarser grid spacings as expected6.

10-day runs: lack of haboobs with parameterised deep convection compensated by stronger SHL (Fig. 4; more LLJs and increased nocturnal monsoon flow7).

Simply using 6-hourly wind analyses would miss both LLJs5 and haboobs8.

Haboob peak

LLJ peak

Haboob peak

Dashed=param.Solid=explicit

Nocturnal monsoon flow

1.5-km domain

Fig. 4. The mean SHL

Runs with parameterised deep convection develop a deeper SHL and so a stronger LLJ (Fig. 3b),

All simulations are within two analyses.

More observations from SHL region needed.

Explicit cold pools may lead to more effective ventilation of SHL.